The present invention is directed to a method for improved chroma-key suppression. Specifically, the present invention is directed to a method for improving the suppression of a halo effect which might otherwise occur in chroma-key processing of images.
It is common in the art of image processing to desire to take two separate images and somehow combine parts of those images to create a third, new image. One common way of creating such a composite image is to take a first image which will constitute a foreground image, and discriminate out those objects which one desires to superimpose upon a background image.
An example of such an operation of creating a composite image will now be described with respect to FIGS. 1A. and 1B. In particular, two images, 101 and 110 are shown in FIG. 1A. It is the goal of the user of an image processing system to create a composite image such as that shown as image 120 in FIG. 1A. For example, a first image 101 can be constituted by an image of an individual standing in front of a single color background. It might be desirable to superimpose the image of the individual over a selection of other images. One such image might be a map of the Untied States roughly shown as element 112 in second image 110. This would commonly be done, for instance, in the reporting of weather information where a weather forecaster""s image is superimposed upon a picture of a map or other information related to the weather forecast. In such examples, the background of the first image is typically a single color such as blue or green. The goal is to create a composite picture such as that shown in 120 where the person 102 is projected or superimposed over the map 112.
In a process that will be described with respect to FIGS. 2A and 2B, in essence, the picture of the individual in the first image is cut out or discriminated from the first image that might be considered a foreground image. That cut out component of the foreground image is then added to the second image or background image. In fact, the second image can be processed to cut out a portion into which the foreground object will be inserted.
One known technique for creating such a composite image is shown in FIG. 2A. This technique involves processing image information in the digital domain. Therefore, keeping with weather map scenario described above, television signal representations are digitized to create a pixel stream associated with each image frame. A pixel representation of the foreground image is first supplied to a discriminator 201. The discriminator analyzes the pixels of the foreground image and detects those that correspond to the background color, that is the color of background 103. A chroma-key signal xe2x80x9cKxe2x80x9d is produced based on the detection of those pixels which correspond to the background color. Typically, this key signal could constitute a stream of 0s and 1s, where 0s indicate that the pixel should be removed because it matches the background color which should be deleted from the image so as to leave only the foreground object. The 1s could correspond to those pixels which do not correspond to the background color and therefore are presumed to be part of the foreground object which is to be supplied in the composite image. The foreground image and the chroma-key signal K are supplied to a multiplier 202 whereby the multiplier output is the discriminated foreground object alone without the background. Furthermore, the chroma-key signal K is used to create a multiplier to be applied to the background image multiplier 203. The result is that the space into which the foreground object is to be inserted is removed from the background image. Thus, the cut-out foreground object and the background image minus the area into which the foreground object is to be inserted are added together by adder 205 to create the composite image. This xe2x80x9chardxe2x80x9d discrimination can result in an abrupt transition that constitutes visibily disturbing aliasing.
It is known in the art to try to perform a softer discrimination than that described above, whereby the processing creates a soft transition from the foreground to the background image. However, one of the drawbacks of known techniques for soft discrimination is that a halo effect is created around the boundary of the foreground object. An example of this is shown in connection with FIGS. 1A and 1B. More particularly, in the dashed box area shown as 121 in FIG. 1A and enlarged in FIG. 1B, one portion of the boundary of the foreground object shown as 125, here the head of the individual, may be subjected to a halo effect because of the fact that the pixels that form the boundary region may have been somewhat close in color to the background 103 but did not correspond to the background color sufficiently to be subjected to the keying operation. That is, those pixels may not have been designated as xe2x80x9c0xe2x80x9d areas that matched the background color which was to be deleted. Therefore, that color which is not truly a part of the object of interest is not deleted from the foreground image in the discriminating operation and carries over into the composite image thereby creating a halo-like effect around the foreground object.
One technique for dealing with this halo-like effect is to modify the first image so as to attempt to force all of the pixels surrounding the foreground object to the same color, thereby changing the pixels in the boundary region to make them more closely correspond to the color of the key patch.
Another technique is shown in FIG. 2B where a suppression operation is supplied to the foreground image before it is multiplied by the key signal K. However, this suppression operation does not typically deal with the extent to which the color or colors around the boundary of the foreground object and the background of the foreground image vary around the boundary of the object and how closely they correspond to the key patch color.
It would be beneficial if any suppression technique or any processing of the foreground image prior to cutting out the foreground object took into account how closely the boundary images corresponded to the key patch while falling outside of the key patch color region.
The present invention improves upon chroma-key suppression techniques by generating suppression signals in accordance with pixel positions relative to a key patch region.
In one embodiment of the present invention, a chroma-key extractor examines the components of a first image. The components can be constituted by pixels. Each of the components or pixels is processed to determine the relative position of the color of that component with respect to a chroma-key patch. Some pixels will be detected to correspond to the key patch area. Others will be discovered to differ substantially from the key patch region. Finally, still others will be found to fall within a transition region adjacent to the key patch region. In accordance with the method of the present invention, suppression signals are determined in accordance with a position of a pixel color in the transition region. For example, if the pixel color falls within a first area of the transition region, the hue of the suppression signal to be applied to the pixel will be selected and then a saturation value will be determined. Similarly, if the pixel is determined to fall within a second area of the transition region, then a saturation component of the suppression signal is selected and a hue for that suppression signal is selected based on the position of the color of the pixel within that second area.
The method of the present invention improves the suppression of the halo-like effect by specifically addressing the phenomenon that arises in the boundary region of the foreground object and the background of the foreground image. More particularly, the present invention provides a more uniform color metric of the image processing operation.